Digital subscriber line (DSL) technology is used to transform an ordinary telephone line (e.g., copper wire twisted-pair) into a broadband communication link. It works by sending signals over the telephone line in previously unused high frequencies.
Over the years, DSL technology has evolved into a family of specific, standardized implementations. These various implementations offer a variety of transmission speeds and transmission distances. It is common to refer to the various DSL implementations that have evolved over the years collectively as xDSL.
Each of the various xDSL implementations typically employs either carrierless amplitude and phase (CAP) modulation or discrete multi-tone (DMT) modulation. CAP modulation is closely related to quadrature amplitude modulation (QAM). CAP modulation produces the same form of signal as QAM without requiring in-phase and quadrature components of the carrier to first be generated. DMT modulation is a modulation method in which the available bandwidth of a communication channel is divided into numerous subcarriers or tones. Each tone of a DMT communication system is capable of acting as a communications sub-channel that carries information between a transmitter and a receiver.
A number of factors determine the performance of the various xDSL implementations. For example, the performance of any of the xDSL implementations is highly dependent on the local loop length (e.g., the length of a twisted-pair circuit between a central office and a customer) and the local loop condition. The local loop condition is affected by several factors such as, for example, line noise. Line noise may corrupt data-bearing signals as the signals travel along the line. As a result, the transmitted data-bearing signals may be decoded erroneously by a receiver because of this signal corruption.
In the case of DMT modulation, for example, the number of data bits or the amount of information that a tone carries may vary from tone to tone, and it depends on the relative power of the data-bearing signal compared to the power of the corrupting signal on that particular tone. A measure of the quality of a signal carried by a tone is the ratio of the received signal strength (power) over the noise strength in the frequency range of operation, or the Signal-to-Noise Ratio (SNR). High SNR results in high signal quality being carried by a tone. Another measure of signal quality is bit error ratio (BER) for a given tone.
In order to account for potential interference on the telephone line and to guarantee a reliable communication between the transmitter and receiver, each tone is typically designed to carry a limited number of data bits per unit time based on the tone's SNR using a bit-loading algorithm. The number of bits that a specific tone may carry decreases as the relative strength of the corrupting signal increases, that is when the SNR is low. Thus, the SNR of a tone may be used to determine how much data should be transmitted by the tone.
It is often assumed that the corrupting signal is a stationary additive random source with Gaussian distribution and white spectrum. With this assumption, the number of data bits that each tone can carry relates directly to the SNR. However, this assumption may not be true in many practical cases and there are various sources of interference that do not have a white, Gaussian distribution. Impulse noise is one such noise source. Time-varying noise is also an effect that makes the stationarity assumption impractical.
Bit-loading algorithms, which are methods to determine the number of bits transmitted per tone, are usually designed based on the assumption of stationary additive, white, Gaussian noise. With such algorithms, the effects of impulse noise and time-varying noise are misestimated resulting in an excessive rate of error. Furthermore, channel estimation procedures can be designed to optimize performance in the presence of stationary, additive, white, Gaussian noise, but are often poor at estimating impulse noise and time-varying noise. Consequently, DSL modem training procedures leave the modem receivers susceptible to impulse noise and time-varying noise.
What are needed are new DSL noise mitigation techniques that overcome the deficiencies noted above.